Colistin, an antibiotic first developed in Japan in the 1940s, was largely abandoned because of its side effects. But in the 1990s it became valuable, being accepted as one of the “last-line” antibiotics reserved to fight rising antibacterial resistance.

But now, a team of scientists at Emory University’s Antibiotic Resistance Center have isolated tiny bacteria in patients that have colisin resistance – a first in America.

The samples were taken from the urine of two patients in Atlanta as part of a CDC Emerging Infections monitoring program. The Klebsiella pneumoniae is part of the Enterobacteriaceae (CRE) family, which is resistant to carbapenem antimicrobials.

The samples originally showed overall typical genetic properties, through testing by broth microdilution and Etest evaluations, report the microbiologists.

But when they took the isolates and grew them out, they revealed themselves to have some individual phenotypes within the overall mixtures that showed resistance to virtually all drugs – including colistin.

The Emory team then attempted an in-vivo model, by infecting mice with both the resistant and non-resistant strains of K. pneumoniae. The mice who had colistin-susceptible germs recovered – but the others with the resistant bacteria all died, add the scientists.

The results should “sound the alarm about a worrisome and underappreciated phenomenon,” conclude the microbiologists.

“We show here that in a mouse model of infection, colistin-heteroresistant CRKP isolates fail colistin therapy,” add the researchers. “This stresses the need for assessing the relevance of colistin heteroresistace on the outcome of colistin therapy in human infection, which has yet to be determined.”

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